With the advent of small, low power stand-alone electronic type cameras for the detection of visible, near-infrared (VNIR) and long-wave infrared (LWIR) images (such as the conventional image-intensified CMOS imager for VNIR image detection and the uncooled micro-bolometer focal plane array imager for LWIR image detection), the number of applications for these types of VNIR or LWIR cameras has expanded to include hand-held, weapon-borne and helmet-mounted applications. However, the conventional VNIR and LWIR cameras each have an image-forming optical systems or objective lens assembly that is typically too long in a field-of-view axis to be suitable for use in hand-held, weapon-borne and helmet-mounted applications.
The present inventor has disclosed, in U.S. patent application Ser. No. 11/539,804, a dual-field-of-view objective lens assembly that is compact and thin in the object side direction such that the objective lens assembly is suitable for use in hand-held, weapon-borne and helmet-mounted applications. However, the disclosed dual-field-of-view objective lens assembly requires separate prism lens groupings with respective entrance pupils for viewing two spectral band images, such as VNIR and LWIR images. Thus, there is a need for a compact lens assembly that employs fewer optical components for simultaneously imaging two or more spectral band images, for example, within the VNIR waveband and the LWIR waveband.
U.S. Pat. No. 7,248,401 (the “'401 patent”) discloses a common-aperture, multispectral objective device that uses a folded beam-splitter and mirror to simultaneously image near infrared (NIR) and LWIR spectral bands. However, the objective device disclosed in the '401 patent when implemented for demonstration to the US Army was found to be susceptible to producing an undesirable forward projection. In addition, the '401 patent's objective device is neither compact in the object direction nor form-fitting to a helmet or other headgear.
Conventional color separating prism assemblies, such as disclosed in U.S. Pat. Nos. 6,667,656; 6,517,209; and 6,078,429, also employ a common aperture for receiving an incoming light beam from an objective lens assembly. However, conventional color separating prism assemblies are not suitable for use as an objective lens or for separating spectral bands within the infrared band for simultaneous imaging. In addition, conventional color separating prisms are neither compact in the object direction nor form-fitting to a helmet or other headgear.
Therefore, a need exists for an objective lens assembly that overcomes the problems noted above and others previously experienced for simultaneously imaging multiple spectral bands, for example in the VNIR and the LWIR bands, for use in hand-held, weapon-borne and helmet-mounted applications.